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Transcript
Summer and Fall Use of Logging Residue
Piles by Female Short-tailed Weasels1
Kimberly A. Lisgo,2 Fred L. Bunnell,2 and Alton S. Harestad3
Abstract
Female short-tailed weasels (Mustela erminea) used piles of logging residue more than
expected and used areas without logging residue less than expected when they were in 3-yearold regenerating aspen (Populus tremuloides) cutblocks during summer and fall (P < 0.001).
Female weasels may prefer piles of logging residue because they offer greater amounts of
food, larger numbers of rest sites, and greater availability of travel corridors. Our data indicate
that food abundance, specifically the southern red-backed vole (Clethrionomys gapperi), best
explains the preferential use of logging residue piles by female weasels. Recommendations
for the management of logging residue piles are discussed.
Introduction
Downed wood provides structure in habitats of mustelids and small rodents,
including secure travel corridors and, in winter, subnivian access and thermal cover.
The importance of downed wood for mustelids, particularly marten (Martes
americana) and fisher (M. pennanti), is well documented (Buskirk and Powell 1994,
Buskirk and Ruggiero 1994, Corn and Raphael 1992) and is expressed in habitat
quality as both amount of cover and available prey. For small rodents, downed wood
can function as travel routes (Carter 1993, Maser and others 1979, Maser and Trappe
1984), provide fungal fruiting bodies for food (Maser and Trappe 1984, Rhoades
1986), and supply nest sites (Weaver 1996). Combined, these factors may encourage
positive association of red-backed voles (Clethrionomys spp.) with downed wood
(Nordyke and Buskirk 1991, Thompson 1996) through increased survivorship and
reproduction. The relationship of weasels to downed wood is less clear. There are
only a few anecdotal accounts that describe use of downed wood by short-tailed
weasels (Mustela erminea). In areas that have been clearcut, logging provides
substantial amounts of residual downed wood that can be dispersed or left in piles.
Gyug (1994) reported that short-tailed weasels used cutblocks with logging residue
piles more than those with none. Because of the shape and size of weasels, it is likely
that the benefits of logging residue piles accrued to small rodents may apply to the
1
An abbreviated version of this paper was presented at the Symposium on the Ecology and Management
of Dead Wood in Western Forests, November 2-4, 1999, Reno, Nevada.
2
Research Technician and Professor, respectively, Center for Applied Conservation Research,
Department of Forest Sciences, University of British Columbia, 3004-2424 Main Mall, Vancouver,
B.C., Canada, V6T 1Z4 (e-mail: kim_lisgo@hotmail.com)
3
Associate Professor, Department of Biological Sciences, Simon Fraser University, 8888 University
Drive, Burnaby, B.C., Canada, V5A 1S6 (e-mail: harestad@sfu.ca)
USDA Forest Service Gen. Tech. Rep. PSW-GTR-181. 2002.
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Use of Logging Residue by Short-tailed Weasels―LIsgo, Bunnell, and Harestad
short-tailed weasel (e.g., travel routes). As well, higher abundance of small rodents
could attract weasels to downed wood as foraging sites.
Our objectives were to determine if female weasels, when they were in
cutblocks, preferred logging residue piles to areas without logging residue piles;
examine factors that could account for such behavior; and propose enhancement of
short-tailed weasel habitat through management of logging residue piles.
Study Area
Our study area (54°58'N, 111°55'W), in the southern portion of the mixedwood
boreal forest, is located at Owl River, approximately 300 kilometers northeast of
Edmonton, Alberta. The mixedwood boreal forest is circumpolar. In Canada, it
extends from northeastern British Columbia, through Alberta and Saskatchewan, to
southeastern Manitoba. The province of Alberta contains approximately 40 percent or
290,000 square kilometers of Canada’s mixedwood boreal forest (Rowe 1972).
The mixedwood boreal region is characterized by a complex mosaic of stands
varying in age, species composition, and structure, interspersed with rivers, lakes,
bogs, and fens. The dominant coniferous tree species are white spruce (Picea glauca)
and black spruce (P. mariana), with balsam fir (Abies balsamea), jack pine (Pinus
banksiana) and larch (Larix laricina) also present (Rowe 1972). Dominant deciduous
species are trembling aspen (Populus tremuloides) and balsam poplar (Populus
balsamifera), with lesser amounts of white birch (Betula papyrifera) (Rowe 1972).
Although relatively pure stands of all these tree species exist, the most common later
seral stages are a mix of species, most often trembling aspen, balsam poplar, and
white spruce. The dominant natural disturbance regime in the mixedwood boreal
forest is fire (Johnson 1992, Rowe 1961). Other disturbances include cutting of
seismic lines for oil and gas exploration, white spruce saw-timber operations,
harvesting of timber for pulp production, windstorms, and pathogens. These
disturbances vary in size and contribute to the complex mosaic of stands that
characterize the landscape.
Our study area is located within the Forest Management Area of Alberta Pacific
Forest Industries Inc. (AlPac) and is located within two townships (Township 69
Range 13 W4M and Township 70 Range 13 W4M), each approximately 10
kilometers by 10 kilometers. The townships are dominated by jack pine stands (1930
origin), black spruce-larch mixed stands (1880 origin), and mature mixed and pure
stands of trembling aspen and balsam poplar (1930-1950 origin). Large wet shrubby
areas of dwarf birch (Betula pumila var. glandulifera), alder (Alnus spp.), and willow
(Salix spp.) also are present. Pure stands of white spruce and mixed deciduousconiferous stands are relatively uncommon. There are several small lakes in the area.
Some agricultural land occurs adjacent to the southern edge of the study area.
Portions of both townships were harvested by clearcutting and partial cutting for
aspen and poplar in winter 1993/94. Based on Alberta Vegetation Inventory mapping
(Alberta Forestry, Lands and Wildlife 1991), cutblocks range in size from 1 to 62
hectares and account for approximately 6.5 percent of the area. During our study,
July to December 1996, cutblocks were densely vegetated with naturally regenerating
aspen and poplar saplings, wild rose (Rosa spp.), herbs, and grasses.
The mean daily temperature for this region is 3.9 to 10.9 degrees Celsius in
summer (May-September) and -1.1 to -22.4 degrees Celsius in winter (October-
320
USDA Forest Service Gen. Tech. Rep. PSW-GTR-181. 2002.
Use of Logging Residue by Short-tailed Weasels―LIsgo, Bunnell, and Harestad
April). The annual mean precipitation is 432.3 millimeters including 131.5
centimeters of snow. Major snowfall months are November to April with a mean
monthly snowfall of 11.9 to 25.0 centimeters during this period (Environment
Canada 1998).
Methods
Radio-tagging and Telemetry
During February-December 1996, we live-trapped weasels in wooden box traps
modified from King (1973) and insulated with cotton. The traps were baited with
bacon or carcasses of white laboratory mice and placed systematically in cutblocks,
but near downed wood or other cover. We used data from four female weasels in this
analysis (females 1 to 4). All four females were adults (> 1 year of age) and were
known to inhabit cutblocks prior to collaring. Each female was fitted with a Holohil
MD2C4 transmitter weighing 2-3 grams, < 4 percent of an adult female’s body
weight. As recommended by King (1989), we held weasels in-hand when attaching
collars rather than anaesthetizing them. We monitored the four females between July
and December 1996 and located them by radio-triangulation (White and Garrott
1990) from fixed telemetry stations. We recorded at least three bearings from
separate stations for each location. After we determined a general location, we
approached the site to confirm activity (i.e., resting versus foraging), specific
location, and habitat type. We flagged the site and returned later to locate it using a
hand-held global positioning system (GPS). The GPS data were corrected to ± 1
meter. Because of equipment problems, telemetry sampling was sometimes erratic. In
general, females were located every 1 to 2 days. Female weasels often damaged their
transmitters, and 1-week periods without locations were not uncommon. We
attempted to obtain the locations over the 24-hour day, but despite our efforts and
because of safety issues working at night, daytime locations dominate our data.
We used variations in signal strength to deduce activity of radio-collared
weasels. Hunting or traveling activity was indicated by changes in the volume of the
radio signal. When inactive, the signal volume remained constant. We identified an
inactive location using zone of receiver (i.e., when the signal could be heard on the
receiver without an external antenna). We moved to the edge of the signal range
where reception was most sensitive to changes in the weasel’s location. We listened
to the signal for 15 minutes. If a change in signal strength was not observed, we
deemed the weasel inactive.
Logging Residue Piles
We defined logging residue piles (LRPs) as the tops and branches of felled trees,
dispersed along the main road, and spurs passing through cutblocks. We determined
the areas of LRPs in four cutblocks used by females 1 to 4. The cutblocks were all
harvested for trembling aspen (Populus tremuloides) during winter, 1993-94, using
dispersed retention with 1-2 percent of the trees left uncut and the logging residue
distributed along access roads in the cutblocks. Approximately 35-68 meters of main
road and spurs occurred per hectare of cutblock.
4
Mention of trade names or products is for information only and does not imply endorsement by the
U.S. Department of Agriculture.
USDA Forest Service Gen. Tech. Rep. PSW-GTR-181. 2002.
321
Use of Logging Residue by Short-tailed Weasels―LIsgo, Bunnell, and Harestad
In May 1997, we walked the perimeter of each LRP taking Universal Transverse
Mercator (UTM) coordinates via hand-held GPS every 5 seconds or approximately
every 2-5 meters. The GPS point data were corrected to ± 1 meter. We converted the
point data into vector polygons using Arc/Info Version 7.1 (ESRI 1997) and imported
the vector polygons into ArcView version 3.1 (ESRI 1996). We calculated the area of
LRPs and cutblock habitat available for use within each female’s home range using a
raster overlay of home range, LRP polygons, and digitized Alberta Vegetation
Inventory maps with a 0.1- x 0.1-meter raster resolution. We defined home range as
“the area traversed by an individual during its normal activities of food gathering and
resting within the period when sampling occurred” (Lisgo 1999). We calculated
home ranges using a fixed kernel estimator based on a 95 percent utilization
distribution (Lisgo 1999).
Use of Logging Residue Piles by Weasels
We tested selective use of LRPs in cutblocks using the log-likelihood ratio (Zar
1984). Availability of cutblocks and LRPs for each weasel was based on their
respective area within a female’s home range. We used both active (i.e., hunting or
traveling) and inactive (i.e., resting or consuming prey) observations in the analyses.
Small Rodent Abundance Estimates
We live-trapped small rodents in two cutblocks, Sites 1 and 2, that 3 female
weasels (females 2 to 4) were known to use. We placed four Multi-capture Tin Cat®
live traps 25 meters apart in a diamond configuration with an additional trap placed
in the centre (five traps in total). We placed six trap arrays in each cutblock: three
trap arrays in areas of LRPs and three trap arrays in areas of no-LRPs. We trapped
each cutblock for three nights from October 4 to 6, 1996. We identified each small
rodent to species and then sexed, weighed, marked it with picric acid, and released it.
Although the rodents were not uniquely marked, we later recorded the number of
marked (previously captured) and unmarked animals during trapping sessions. We
estimated abundance for four areas: 1) Site 1 areas of LRPs; 2) Site 1 areas of noLRPs; 3) Site 2 areas of LRPs; and 4) Site 2 areas of no-LRPs. In each case, data
from the three tray arrays were pooled.
We used the Schumacher and Eschmeyer method, a version of the Schnabel
method, to estimate the abundance of small rodents with 95 percent confidence limits
(Krebs 1989). This method estimates the abundance for a closed population (i.e.,
population size is constant without recruitment or loss). It is appropriate for this study
because the trapping occurred over a short period. The Schumacher and Eschmeyer
method distinguished two types of individuals: 1) marked individuals caught
previously at least once; and 2) unmarked individuals not caught previously. We
calculated 95 percent confidence limits for each population estimate using a Poisson
distribution as recommended for cases in which total number of recaptures is less
than 50 (Krebs 1989).
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Use of Logging Residue by Short-tailed Weasels―LIsgo, Bunnell, and Harestad
Results
Area of Logging Residue Piles and Their Use by Weasels
On average, 57 percent (112/198) of the females’ locations were in cutblocks
and 43 percent (86/198) in uncut forest and shrubby habitats. Within cutblocks, the
area of LRPs in female home ranges varied from 2.03 to 3.93 hectares (table 1).
When in cutblocks, all four females preferred areas of LRPs to areas without LRPs (P
< 0.001, table 2). Collectively, observations of inactive females in areas with LRPs
and without LRPs account for only a small proportion of the data, 11 percent and 18
percent, respectively (table 2).
Table 1―Cutblock area found within the home range of each female weasel and area within
the cutblock designated as areas with logging residue piles (LRPs) or without logging residue
piles (no-LRPs) based on GIS analyses.
Weasel
Female 1
Female 2
Female 3
Female 4
Home range
size
(hectare)
82.6
94.8
65.5
76.8
Cutblock
area
(hectare)
24.43
34.70
32.09
23.53
Area of noArea of LRPs Percentage LRPs
LRPs (hectare)
(hectare)
in cutblock area
22.40
32.59
28.16
21.21
2.03
2.11
3.93
2.41
8.3
6.1
12.2
10.2
Table 2―Selective use of logging residue piles within cutblocks by female short-tailed
weasels in the mixedwood boreal forest of central Alberta, based on the log-likelihood ratio.
Values in parentheses are the number of inactive observations in areas with logging residual
piles (LRPs) and without logging residue piles (no-LRPs).
Weasel
Female 1
Female 2
Female 3
Female 4
Observed
Observed
Expected
Expected
locations in locations in locations in locations in
LRPs
no-LRPs
LRPs
no-LRPs
11 (4)
7 (0)
1.5
16.5
14 (0)
18 (5)
2.0
30.0
21 (2)
34 (7)
6.7
48.3
7 (0)
6 (0)
1.3
11.7
Loglikelihood
ratio
31.82
36.09
24.10
15.55
P
< 0.001
< 0.001
< 0.001
< 0.001
Abundance of Small Rodents
We caught three species of small rodents: southern red-backed voles (C.
gapperi), meadow voles (Microtus pennsylvanicus), and deer mice (Peromyscus
maniculatus) (table 3). We captured relatively few meadow voles with most
individuals appearing in traps on the last trap night. Estimates of abundance for redbacked voles showed a trend for greater numbers in areas of LRPs, while estimates of
abundance for deer mice did not differ between areas of LRPs and no-LRPs (fig. 1).
USDA Forest Service Gen. Tech. Rep. PSW-GTR-181. 2002.
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Use of Logging Residue by Short-tailed Weasels―LIsgo, Bunnell, and Harestad
Table 3―Species and the number of captured and recaptured small rodents in areas with
logging residue piles (LRPs) and without logging residue piles (no-LRPs) for Sites 1 and 2
from October 4-6, 1996 in the mixedwood boreal forest of central Alberta.
Site 1
No. captured No. recaptured
No. captured
Site 2
No. recaptured
Treatment
Species
no-LRPs
red-backed vole
deer mouse
meadow vole
33
33
2
11
16
0
13
38
0
3
14
0
LRPs
red-backed vole
deer mouse
meadow vole
42
35
5
7
14
0
29
31
2
8
11
0
18 0
17 0
16 0
15 0
14 0
13 0
estimate of abundance
e s tima te o f a b un d an c
12 0
n o -LRP s
LRP s
11 0
10 0
90
80
70
60
50
40
30
20
10
0
site 1
site 2
red-backed vole
site 1 site 2
deer mouse
s p e c ies o f s ma ll ro d en ts
species of small rodents
Figure 1―Abundance estimates of southern red-backed voles and deer mice with 95
percent confidence limits based on the Schumacher and Eschmeyer method (Krebs
1989), October 4-6 1996, in areas of no logging residue piles (no-LRP) and logging
residue piles (LRP), in regenerating aspen cutblocks (harvested winter 1993/94), in
the mixedwood boreal forest of central Alberta.
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Use of Logging Residue by Short-tailed Weasels―LIsgo, Bunnell, and Harestad
Discussion
There are two features to our study that could potentially bias our results. First,
the high proportion of daylight locations in our data set may bias our estimates of
habitat use. However, in summer, North American and European weasels are
predominantly diurnal in activity (Debrot and others 1985, Erlinge 1979, Samson and
Raymond 1995). Second, it is also possible that radio-tagging compromised the
behavior of collared weasels and that these weasels do not accurately represent the
population. White and Garrott (1990) stress the importance of testing the effect of
radio-tagging on study animals. Lisgo (1999) found that collared weasels did not
suffer nutritionally; however, we do not know if the normal hunting behavior of
weasels was affected by the presence of a radio-collar. For example, the increased
diameter of a weasel’s neck caused by a radio-collar may have limited the size of
mouse and vole tunnels that could be explored.
In the mixedwood boreal forest of Alberta, four female weasels preferentially
used areas with LRPs in cutblocks during summer and fall. They spent three to seven
times more time in LRPs than expected. Although our sample size was small, all
females showed similar behavior. Also, our results are consistent with the behavior of
weasels in southern British Columbia during winter (Gyug 1994). We think that
female weasels may selectively use areas with LRPs for three reasons: the amount of
food; availability of rest sites; and availability of travel corridors with security cover.
Our findings suggest that abundance of food, specifically red-backed voles, best
explains the preferential use of LRPs by female weasels.
We think that food resources promoted use of LRPs because LRPs had a higher
abundance of prey. Abundance of prey, at least for Site 1 and particularly red-backed
voles, tended to be higher in LRPs than in areas without LRPs. Deer mice were
distributed evenly throughout both cutblocks. Lisgo (1999) found red-backed voles
were an important component of the weasel’s diet, significantly preferred over deer
mice (P < 0.05). Higher abundance of prey, especially red-backed voles, in LRPs
would reduce hunting effort for weasels and make these areas attractive as foraging
sites.
Other studies also have found positive associations between red-backed voles
and downed wood (Nordyke and Buskirk 1991, Thompson 1996). In the mixedwood
boreal forest, Roy and others (1995) found both red-backed voles and deer mice
positively associated with downed woody material. Roy and others (1995) also
concluded that deer mouse populations are flexible in their habitat requirements
while red-backed vole populations are less so and “select areas of dense shrubs with
abundant downed woody material.” The flexible habitat requirements of deer mice
explain their even distribution throughout the cutblocks. The red-backed voles
positive association with downed wood explains their trend for greater abundance in
LRPs.
In our study area, Weaver (1996) found that red-backed voles and deer mice
nested beneath LRPs, rotting downed wood, and the base of stumps and snags in
cutblocks. Female weasels also rested in LRPs; thus, rest site availability may also
encourage the use of LRPs by female weasels. Weasels rely on natural cavities or the
nests of other species for rest sites (King 1989). Hence, we expect more resting
activity by female weasels in LRPs due to a greater availability of nests, which may
be associated with the higher abundance of small rodents. It does not appear,
however, that LRPs are necessary for female weasels to find suitable rest sites in
USDA Forest Service Gen. Tech. Rep. PSW-GTR-181. 2002.
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Use of Logging Residue by Short-tailed Weasels―LIsgo, Bunnell, and Harestad
cutblocks because female weasels also rested at the bases of stumps and snags in
portions of cutblocks without LRPs (Lisgo 1999).
The response of weasels to LRPs may be an anti-predator behavior by which
they choose travel routes with the least risk of predation (Harestad 1991). Although
LRPs may function as secure travel corridors for female weasels, we think it unlikely
that this would be the sole attractant for females because dense vegetation throughout
the cutblocks provided alternative sources of security and thermal cover. However,
LRPs may facilitate movements of weasels by providing travel routes less hindered
by dense vegetation while providing security cover.
Despite our small sample size, this information on habitat use is valuable to
forest managers. There are few data on the ecology of the short-tailed weasel in
North America or the effects of silvicultural practices on the behavior of weasels.
However, caution must be used when applying our findings to weasels outside our
study area, and in particular, to male weasels. The behavioral response of male
weasels to the presence of logging residue piles is not known and, based on work in
our study area, males use the landscape differently than females (Lisgo 1999).
Caution is also necessary when interpreting the responses of small rodents to
LRPs. First, the level of replication was small and our findings for small rodents may
be restricted to the cutblocks that we sampled. Second, the sampling period was short
and may not be representative of the full 6 months during which female weasels were
observed.
Management Recommendations
Female weasels preferred areas with LRPs that were used for resting, hunting,
and possibly travel routes. Small rodent abundance, in particular red-backed voles,
suggests that prey abundance may be a factor promoting the use of LRPs by females.
At the time of our study (3 years post-harvest), LRPs were not essential for providing
suitable rest sites because female weasels also rested in portions of cutblocks without
LRPs. Based on our findings, LRPs tend to support a higher abundance of red-backed
voles and appear to be a beneficial attribute of 3-year-old regenerating aspen
cutblocks in our study area.
Because female short-tailed weasels prefer logging residue piles, timberharvesting practices could be altered to provide residual downed wood. Besides
amount, distribution of logging residue piles may be important to supplying suitable
habitat for both small rodents and weasels. Logging residue can be dispersed, piled,
or removed from a site depending on silvicultural prescriptions and harvesting
methods. We recommend that LRPs be retained and distributed linearly along main
roads and spurs to ensure they occur in home ranges of several female weasels. Many
other wildlife species inhabit cutblocks and the habitat quality for some likely would
be diminished if this recommendation was applied throughout the landscape. To
sustain all wildlife species, the same practices should not be used everywhere
(Bunnell and others 2002). A diverse approach, managing for multiple species, is
more appropriate for sustaining wildlife in the mixedwood boreal forest. Moreover,
the data provided are based on a 6-month study, and weasel home ranges fluctuate
with prey abundance (Erlinge 1977). Declines or increases in abundance of prey may
result in expansion or contraction of home range size and could change use of
habitats by weasels. Also, over time, the reduction of LRPs through decay may lower
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Use of Logging Residue by Short-tailed Weasels―LIsgo, Bunnell, and Harestad
the suitability of cutblocks as habitat for weasels. Lastly, because of the mixedwood
boreal forest’s diverse landscape, the habitats studied are not representative of the
entire mixedwood boreal forest. Recommendations suitable for the forest types in our
study area may be inappropriate for other locations, such as those dominated by older
mixedwood stands.
Acknowledgments
Funding and logistical support for this project were provided by Alberta Pacific
Forest Industries Inc., Natural Sciences and Engineering Research Council of
Canada, Challenge Grant in Biodiversity Program (Department of Biological
Sciences, University of Alberta, and the Alberta Conservation Association), the
Sustainable Forest Management Network, and the Alberta Sport, Recreation, Parks,
and Wildlife Foundation. We thank Dr. Rich Moses and Robin Weaver from the
University of Alberta and Dr. Gitte Grover from Alberta Pacific Forest Industries for
their support while in the field. A special thanks to all field assistants for their hard
work: Josephine Osborne, Mandy Kellner, Robin Weaver, Kelly Scott, Lindsay
Tomyn, Gloria Lisgo, and Zak Calloway. The manuscript benefited from reviews by
Rich Moses and Doug Steventon. This is publication R-38 of the Centre for Applied
Conservation Biology, University of British Columbia.
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